A conventional laser device is described in Japanese laid open No. 56-115591(115591/1981), the laid open date of which is Sept. 10, 1981. The laser device comprises a DC power source having a plurality of output terminals for a plurality of different predetermined voltages; a plurality of semiconductor switches for selecting one of the different predetermined voltages; means for selecting one of the semiconductor switches to be turned on; a capacitor to be charged through a selected switch from the DC power source; a flash lamp for a light source to be turned on in accordance with the charged voltage of the capacitor; and a laser rod to be excited in accordance with light from the flash lamp.
In operation, one of the semiconductor switches is turned on, depending upon an energy level of laser light which is required in laser machining, so that the capacitor is charged to a predetermined level of voltage in accordance with a charging voltage from the corresponding output terminal of the DC power source. The flash lamp is turned on in accordance with the charged voltage of the capacitor so that light radiates from the flash lamp thereby exciting the laser rod to emit laser light. The laser light thus emitted is focussed through an optical system on the surface of a hard article, such as metal, so that laser machining or drilling an aperture, cutting, welding etc. Of the hard article are carried out. In the laser device, it is possible to change the energy level of the laser light at high speed without difficulty simply by turning on one of the semiconductor switches.
According to the foregoing conventional laser device, however, there is a disadvantage that laser light is affected by thermal lens effect of the laser rod. In more detail, the laser rod functions as a concave lens during the starting period of excitation when a flash lamp for exciting the laser rod is turned on and off with a shorter cycle than the thermal relaxation time of the laser rod. This may happen because the temperature of the laser rod initially is higher in its cross section at its inner peripheral portion than at its central portion. On the other hand, the laser rod functions as a convex lens after it reaches thermal equilibrium because the temperature of the laser rod is higher in its cross section at its central portion than at its inner peripheral portion. For this reason, a laser rod is generally cooled from its outer periphery by water.
Therefore, defective laser machining of an article occurs due to fluctuation of the energy density of produced laser light as a result of the thermal lens effect occuring during excitation of a laser rod as mentioned above.
Additionally, another conventional laser device includes a shutter means provided on a laser light axis to shut off laser light from a laser rod before the rod reaches thermal equilibrium and to pass laser light thereafter to permit laser machining to run.
The latter laser device, however, has the disadvantage that laser light is not effectively utilized and the life of a flash lamp is thereby shortened.